This invention relates generally to fabricating hollow airfoils and more particularly concerns a method of producing lightweight, high-strength hollow airfoils using diffusion bonding and superplastic forming techniques. This method is particularly useful in making hollow titanium aircraft engine blades for integrally-bladed rotors.
Hollow airfoils are utilized in gas turbine engines to reduce the weight of the engine. Weight reduction becomes vitally important as the gas turbine engine thrust is increased. One of the ways that thrust is increased is by increasing engine size. As the engine size increases, individual part size and part weight also can increase. In the past, weight reduction has been accomplished by developing strong, lightweight alloys. For airfoils such as aircraft engine blades, which typically are solid for smaller engines, the increase in size precludes the use of solid airfoils because of the substantial weight gain, even when lightweight materials such as titanium alloys are used. In order to produce useful but light aircraft engine blades without incurring unacceptable weight penalties, it is necessary to manufacture either composite blades or hollow metallic blades.
Hollow metallic blades are frequently formed by taking advantage of the superplastic forming and diffusion bonding behavior of certain metals. Superplastic forming is a technique that relies on the capability of certain materials, such as titanium alloys, to develop unusually high tensile elongation with a minimal tendency towards necking when submitted to coordinated time-temperature-strain conditions within a limited range. Superplastic forming is useful in producing a wide variety of strong, lightweight articles.
Many of the same materials used in superplastic forming are also susceptible to diffusion bonding. Diffusion bonding is a process which forms a metallurgical bond between similar parts which are pressed together at elevated temperature and pressure for a specific length of time. Bonding is believed to occur by the movement of atoms across adjacent faces of the parts. Diffusion bonding provides substantial joint strength with little geometrical distortion and without significantly changing the physical or metallurgical properties of the bonded material.
It has long been desirable to fabricate various aircraft components, such as door panels and wing flaps, as hollow bodies. The benefits of such include a substantial reduction in weight which provides improved fuel efficiency and increased thrust-to-weight ratio. Despite the increasing popularity in applying diffusion bonding and superplastic forming (DB/SPF) techniques to the manufacture of aircraft components, there are many critical problems to overcome in successfully forming a hollow airfoil. Parts formed using DB/SPF techniques have very complex geometries, exhibit highly non-linear material behavior, and are subject to large irreversible strains. Thus, there exists the possibility of many deformation-induced instabilities, such as necking, grooving, buckling and shear localization, which substantially weaken the structural integrity of the part.
The stringent requirements for both the external aerodynamic shape and internal structure of hollow airfoils present another problem in the manufacture of such parts. In order to produce the desired final shape and thickness, the proper in-process shape (i.e., the shape and size of a part prior to superplastic deformation) must be known.
Accordingly, there is a need for a method of manufacturing hollow airfoils having aerodynamic shapes and complex geometries using superplastic forming and diffusion bonding. Particularly, there is a need for such a method which can achieve the final desired shape and thickness without compromising the physical and metallurgical properties of the bonded metal while alleviating problems of deformation-induced instabilities.